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Blog
How do I install a pneumatic differential pressure transducer?
July 16, 2007
This question was posted on control.com by an anonymous user:
“You are working with a pressurized tank that contains a liquid with SG = 1.0, and level measuring pressure taps that are 100 inches apart. A pneumatic-differential pressure transmitter is used to measure level. The leg that connects the top of the tank to the transmitter is filled with tank liquid. The top tap should be connected to:
1 - The low-pressure tap on the transmitter
2 - The high-pressure tap on the transmitter.
3 - The high-pressure tap but through a seal chamber
Please tell me which one is the right answer.”
You are already in trouble if you’re using a pneumatic differential pressure transducer and have your top leg filled with fluid. The word “pneumatic” implies that the transducer was designed to work with air (or at least some reasonably non-reactive gas) on both sides of the diaphragm. Saying that you’ve got the top leg filled with fluid implies you’ve already violated this requirement and (perhaps) toasted the transducer.
By a “seal chamber” I assume you mean an isolating diaphragm used to transmit pressure to the sensing diaphragm while protecting it from tank fluid. It would make no sense to go through the time, effort and expense to build such a system, when industrial-quality pressure transducers are commercially [www.omega.com/guides/pressXducers.html] available that either don’t need them or have them already built in.
This falls under my oft-quoted rule: Never build anything you can by off-the-shelf! Why invest the non-recurring engineering cost yourself, when somebody else has already amortized it over production-volume quantities.
So, let’s throw away the ruined transducer and start over with a wet-diaphragm/wet-fitting transducer. I’ll suggest a PX771A-100WCDI industrial differential pressure transducer available from Omega Engineering, but most any other wet/wet differential pressure transducer with a range at or above 0-100 inH2O will do. Just make sure it’s burst pressure exceeds the maximum tank pressure.
Now that we’ve got a proper transducer for the application, the next problem is plumbing it in so that it’s pretty much guaranteed to work with a minimum of fuss. We do that by connecting the differential pressure transducers high side into the highest point we can reach and the low side into the lowest point we can reach.
Keeping the tubes connecting the transducer ports filled provides a stable reference for the level measurement.
Yes, this is counterintuitive, but you’ll see why it works.
The pressure transducer should locate somewhere between these two levels. Note that we make sure that the tubes leading from the pressure transducer to the connection points must be completely filled and remain so to make accurate measurements. It is important to ensure that they stay filled at all times and that there are no bubbles in those lines.
Carefully analyzing the hydrostatics in this system shows that the actual fluid level in the tank (L) is given by the formula:
L = A + B – P/S,
where P is the differential pressure and S is the process fluid’s specific gravity. As the textbooks like to say: “Proof of the formula is left to the reader for an exercise.” To see if you're right, download the full analysis here.
From the diagram, you can see that the reason for connecting the transducer’s high-pressure port to the tank top is that the fluid column below the pressure transducer always acts to reduce the pressure at the transducer’s low port. No matter where the transducer is located, the pressure in the port connected to the top always works out to be lower than that at the port connected to the bottom. The reason you always want both tubes filled is to maintain those fluid columns constant.
Posted by Charlie Masi on July 16, 2007 | Comments (0)